Electronic states of cobalt ions in eubaco2o5 + δ layered cobaltites

The oxygen K-edge X-ray absorption spectra in EuBaCo2O5.52 ± 0.02 and EuBaCo2O5.24 ± 0.02 cobaltites are measured at temperatures of 300 and 440 K, which are below and above the metal–insulator transition temperature, respectively. According to these spectra, the substitution of Co2+ ions for some Co3+ ions with a decrease in the oxygen content in the chemical formula of a cobaltite and, hence, an increase in the relative fraction of CoO5 pyramids with respect to the number of CoO6 octahedra leads to an increase in the band gap by about 0.3 eV. The band structure of EuBaCo2O5.5 is calculated using the method of linearized muffin-tin orbitals in the local density approximation taking into account the local Coulomb interaction. It is found that the low-spin state of Co3+ ions occurs in CoO6 octahedra in EuBaCo2O5.5, whereas the high-spin state of Co3+ ions is typical of cobalt ions in pyramids.